下列关于数据库数据模型的说法中，哪一个是不正确的

A．任何一张二维表都表示一个关系

B．层次模型的数据结构是有向树结构

C．在网状模型中，一个子结点可以有多个父结点

D．在面向对象模型中每一个对象都有一个惟一的标识

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The most important drink is water. Not many people understand this but it is quite t  1 . People can l  2   for a long time without food, but they will die if two or three days w  3   water. In h  2   weather, many people do not drink e  5   water, because they don’t know how much water they n  6  . After t  7   exercise, people need more water.

The water in man’s body is m  8   more than other matters(物质). We’d better have enough water, or we may get i  9  . Water is n  10   to all the living things. There will be no l  11   if there is no water.

So we must save water and don’t w  12   water.